1. Field of the Invention
The present invention relates to a piezoelectric power generation device, and more particularly to a piezoelectric power generation device which can generate electric power with vibrations (oscillations) at low frequencies of not higher than about 20 Hz, for example.
2. Description of the Related Art
Hitherto, there is known a piezoelectric power generation device which causes vibrations upon application of accelerations or strains and generates electric power based on the piezoelectric effect. When the piezoelectric power generation device is employed as a power supply, another additional power supply, such as a battery, is no longer required. Accordingly, the above-mentioned piezoelectric power generation device is suitably used as, e.g., a power supply for various sensors which are employed in situations where vibrations or strains are applied.
One example of the above-mentioned piezoelectric power generation device is disclosed in Japanese Patent No. 3170965. FIG. 19 is a perspective view of a piezoelectric power generation device described in Japanese Patent No. 3170965. A piezoelectric power generation device 100 illustrated in FIG. 19 is a unimorph-type piezoelectric power generation device in the form of cantilevered beam. More specifically, as illustrated in FIG. 19, the piezoelectric power generation device 100 includes a piezoelectric element 102. A metal plate 103 is joined to the piezoelectric element 102. Respective one ends of the piezoelectric element 102 and the metal plate 103 are fixed to a case 101. A weight 104 is attached to the other ends of the piezoelectric element 102 and the metal plate 103. Upon acceleration being applied to the piezoelectric power generation device 100, the weight 104 is caused to vibrate. With the vibration of the weight 104, tensile stress and compressive stress are alternately imposed to the piezoelectric element 102. As a result, the piezoelectric element 102 generates electric power.
In the piezoelectric power generation device 100, the resonance frequency of the piezoelectric element 102 is correlated to both the length of the piezoelectric element 102 and the weight of the weight 104. More specifically, as the length from the fixed end to the free end of the piezoelectric element 102 increases, the resonance frequency of the piezoelectric element 102 lowers. Also, as the weight of the weight 104 increases, the resonance frequency of the piezoelectric element 102 lowers.
In order to generate electric power with vibrations at low frequencies, therefore, it is required to lower the resonance frequency of the piezoelectric element 102 by increasing the length of the piezoelectric element 102 and the weight of the weight 104. Thus, trying to generate electric power with vibrations at low frequencies causes a problem that the size of the piezoelectric power generation device 100 necessarily increases. In particular, the frequency of a motion of a human body, e.g., walking, is usually as very low as about 20 Hz or less. Accordingly, when the low-frequency vibration provided by the motion of the human body is utilized to generate electric power, a problem arises in that the piezoelectric power generation device 100 is required to have a very large size.